Method and device for noise reduction in a pulse-width controlled image display device

ABSTRACT

The invention relates to a method and a device for noise reduction in a pulse-width controlled image display device. A monitoring of whether the brightness of several sequential image points is brighter than a given brightness threshold value occurs. If not the case, the delayed but otherwise unchanged input signal is supplied to the display. If, on the other hand, the brightness of several sequential image points is less than the given threshold brightness value, a mean value generation for the brightness values of several sequential image points occurs and said mean value is supplied to the display.

[0001] The invention concerns a method and a device for noise reductionin a pulse-width-controlled image-display device.

[0002] For example, such a method and such a device are used in plasmadisplays, which will complement or replace the color tubes still used inhigh-quality television equipment at the present time. In connectionwith color tubes, the user of high-quality television equipment has beenaccustomed to flicker-free reproduction since the end of the 80s due tothe 100 Hz technology.

[0003] A plasma display is known from the journal, Radio FernsehenElektronik RFE, No. 2, 1997, pages 18-20, which consists of two glassplates with electrodes arranged in a matrix, between which a noble gasmixture is located. The image information in plasma displays is notrepresented by lines, as in the case of the cathode-ray tubes, but as awhole image. Since, in a plasma display, the individual image pointscannot be turned on and off individually at arbitrary times, theactivation of the image point must occur in an activation pass for theentire display.

[0004] The start-up of a plasma display occurs in several phases: anaddressing- or initialization phase, a holding- or activation phase anda turn-off phase.

[0005] In the addressing- or initialization phase, all cells of theplasma display which are to be activated in the subsequent holding- oractivation phase are precharged. In the last step, the turn-off phase,the precharged cells are discharged and the image information is turnedoff.

[0006] The time interval available for the representation of a TV imageis divided into time intervals of different duration or differentweighting, while a predetermined activation sequence is chosen as afunction of the brightness of a particular image point. This correspondsto lighting up of the particular image point once or several timesduring the image presentation in the available time interval, where apredetermined time duration is assigned to each lighting up process.

[0007] Such known plasma displays are sold on the market, for example,by the companies Fujitsu and NEC.

[0008] From DE 198 33 597 A1, a method and a device are known forreduction of flicker in pulse-width-controlled image-display devices,especially in a color plasma display. Such a color plasma displayserves, for example, for the representation of TV images. The colorplasma display is controlled with a pulse-width modulator, whereas, fortriggering, the duration of a TV image is divided into a number ofpartial images or partial time intervals, which are shown sequentially.In order to reduce flicker, especially to reduce a 50 Hz flicker, thesequence of the partial time intervals and/or activation sequences ofthe partial time intervals is predetermined so that the flickering ofthe images to be presented is minimum.

[0009] Furthermore, a motion-detector-dependent change of the sequenceof time intervals is known. When motion is present, the sequence of timeintervals is chosen so that motion artifacts are avoided. Otherwise thesequence of the tine intervals is such that 50 Hz flicker disturbancesare reduced.

[0010] Furthermore, it is already known in connection with plasmadisplays that one can determine the brightness of an image to berepresented, derive a maximum permissible illumination duration for eachpartial time interval of the image to be represented from the determinedbrightness value and change the maximum permissible illuminationduration for each of the partial time intervals in case of change of thedetected brightness value. This change is done in such a way that, at adetermined dark image content or low brightness value, the maximumpermissible illumination duration is increased in each of the partialtime intervals by the same time duration. However, on the other hand, ifit is found that, in the brightness determination of the image to berepresented, an overall bright image content is present, then themaximum permissible illumination duration of each partial time intervalis reduced by a duration which is the same for all partial timeintervals.

[0011] A disadvantage of this procedure consists in the fact that thecontrast of the image to be produced is reduced, because, if thebrightness of the image to be represented is high, the time duration forthe representation of bright image components is reduced, and, if thebrightness of the image to be represented is low, dark gray imagecomponents are represented as light gray, because these are pulled up,that is, are lit up longer as a result of the application of a constantoffset described above.

[0012] Furthermore, a method is known from German Patent Application 10009 858 for improving the contrast of a pulse-width-controlled imagedisplay device. In this method, a change of the maximum permissibleillumination duration for the partial time intervals is carried out insuch a way that the maximum permissible illumination duration issubjected to smaller changes for partial time intervals with lower valuethan the maximum permissible illumination duration for partial timeintervals with a higher value.

[0013] Starting from this state of the art, the task of the invention isto provide a new method and a new device in which noise is reduced.

[0014] This task is solved by a method with the characteristics given inClaim 1 and/or by a device with the characteristics given in Claim 4.

[0015] Advantageous embodiments and further developments of theinvention follow from the dependent Claims.

[0016] The advantages of the invention consist in the fact that noise isreduced in the dark areas of a displayed image. Thus, the circumstanceis taken into consideration that, in the case of pulse-width-controlledimage-display devices, especially in plasma displays, the noise thatoccurs in the dark areas of the image is especially disturbing. This tobe attributed to the fact that short partial time intervals are assignedto dark image areas. For example, when, due to the noise, not only thepartial time interval is activated which is assigned to the LSB of amultibit word, but also the neighboring partial time interval LSB+1,then this means already a doubling of the brightness, which is reportedby the human eye and is found disturbing.

[0017] This applies especially when, in order to obtain artificialincrease of the number of gray stages in the sense of a “ditheringprocess”, both LSB as well as LSB+1 are divided into four neighboringpixels of the image to be shown. In this case, any noise present causesadditional spatial effects in the abovementioned four-block, whichappears disturbing to the human eye.

[0018] With the aid of the claimed replacement of image points with lowbrightness values with average brightness values, the noise that occursin the dark areas of the image is reduced. In the bright areas of theimage, no change of the image signal occurs, since the noise in thebright areas of the image appears to be less disturbing.

[0019] The claimed method can be used both in the sense of aone-dimensional filtering as well as of a two-dimensional and eventhree-dimensional filtering. In the case of two-dimensional filtering,only additional line memories, and in case of three-dimensionalfiltering, image memories are additionally provided.

[0020] Optionally, the filtering that was performed can extend to imagepoints which lie in the transition region between filtered andunfiltered data. As a result of this, the transitions are made softer.

[0021] Furthermore, the number of sequential image points, which arechecked in the first step, can be chosen to be of different size. Thisalso applies to the number of image points which are used to form themean value. Naturally, in these cases, the delay time of the delaymember lying in the path of the unfiltered signal must be adjustedappropriately.

[0022] Further advantageous properties of the invention follow from theexplanation of a practical example with the aid of figures. Thefollowing are shown:

[0023]FIG. 1 is a sketch explaining the triggering of apulse-width-controlled image-display device,

[0024]FIG. 2 is a block diagram explaining the practical example of theinvention, and

[0025]FIG. 3 is a time diagram to illustrate the mode of functioning ofthe invention.

[0026]FIG. 1 shows a sketch which explains the triggering of apulse-width-controlled image-display device, for example, of a plasmadisplay. The time interval of 20 milliseconds available for therepresentation of a TV image is divided into partial time intervals SF1,SF2, . . . , SF6. Each of these partial time intervals has a differentduration or weighting G, which are shown in FIG. 1, always above thelittle boxes representing the partial time intervals. During thesepartial time intervals, depending on the brightness value of aparticular image point, a predetermined activation sequence is selected.This corresponds to lighting up the image point once or several timesduring the time interval available for representing the image, and apredetermined time duration is assigned to each lighting up.

[0027] For example, in the case of dark image points, only one lightingup occurs during the partial time interval SF1, which corresponds to theLSB of a video signal. If, due to noise, lighting up of SF2 also occursduring the partial time interval, which, in comparison to SF1, has alength which is approximately twice as long or weighted double, thenthis means an almost doubling of the brightness, which is detected bythe human eye as disturbing.

[0028] In order to avoid such disturbances caused by noise, which occurabove all in dark areas of the image, a signal processing is performedas it is described in FIG. 2 with the aid of a practical example of theinvention.

[0029]FIG. 2 shows a block diagram to explain a practical example of theinvention. A video signal in the form of a signal data stream isintroduced to input E, which corresponds to the brightness signals ofsequential image points of the image to be displayed on the plasmadisplay. This video input signal is divided into three signal paths.

[0030] In the upper signal path, it arrives to a test unit 1, whichchecks if the brightness of three sequential image points is alwayssmaller than a predetermined brightness threshold value. If this is notthe case, then test unit 1 generates a control signal for a switch 4through which switch 4 is brought into its upper switching position. Onthe other hand, if the brightness of three sequential image points issmaller than the predetermined brightness threshold value, then the testunit 1 produces a control signal for switch 4 through which switch 4 isbrought into its lower switching position.

[0031] In the middle signal path, a delay member 2 is provided in whichthe input data stream is slowed down by the duration of two imagepoints. The output signal of the delay member 2 is introduced to theupper input of switch 4.

[0032] In the lower signal path, the input signal is passed through afilter 3, in which a linear or weighted mean value formation of threesequential image points occurs. The output signal of filter 3 isintroduced to the lower input of switch 4.

[0033] The output of switch 4 is connected to a plasma display 5, wherethe signal representation occurs.

[0034] When the brightness of three sequential image points are allbelow the brightness threshold value, then, in the circuit shown, thelower signal path is switched so that a mean signal is introduced todisplay 5 and then, when the brightness of three sequential image pointsdoes not lie below the brightness threshold value, the middle signalpath is switched, so that the delayed but otherwise unchanged inputsignal is further conducted to display 5.

[0035] The test described above and the signal processing performed as afunction of the test result, is repeated from image point to imagepoint.

[0036]FIG. 3 shows a time diagram in which the functioning of theinvention is illustrated. In FIG. 3a, the signal data stream present atthe input E is shown, where the brightness values of the sequentialimage points are given in the hexadecimal form. This signal data streamis subdivided into three regions B1, B2, B3 for illustration, where thebrightness values of the image points in regions B1 and B3 are greaterthan the predetermined brightness threshold value, which, in the presentpractical example, is at 30 in the hexadecimal representation and wherethe brightness values of the image points in area B2 are smaller thanthe abovementioned brightness threshold value.

[0037]FIG. 3b shows the signal data stream present at the outlet of thedelay member 2, which is delayed in comparison to the input signal datastream by the duration of two image points.

[0038]FIG. 3c shows the delayed, filtered data stream, as it is presentat the output of filter 3 and FIG. 3d shows the data stream present atthe output of switch 4. It can be seen from the latter that thebrightness values in the regions B1 and B3, apart from the delay by theduration of two image points, agree with the corresponding brightnessvalues of the input data stream. The brightness values in region B2 are,on the other hand, replaced according to the present invention by meanvalues, which were determined by the brightness values of threesequential image points.

[0039] By forming this mean value, which is performed only in the darkareas of the image, the noise in these dark image areas is reduced.

1. Method for noise reduction in a pulse-width-controlled image-displaydevice, where the time interval available for representing the image isdivided into sequential weighted partial time intervals, which form asignal data stream, and the brightness signals present in the form ofmultibit words belonging to the image points of the image to berepresented, are produced by changing to activation sequences assignedto the partial time intervals, characterized by the fact, that a) in afirst step, it is examined if the brightness of several sequential imagepoints is greater than a predetermined brightness threshold value, b) ina second step then, when the brightness of several sequential imagepoints is always smaller than the predetermined brightness thresholdvalue, a mean value of the brightness value is formed from these severalsequential image points and this mean value is used in the signal datastream and then, when the brightness of several sequential image pointsis not always smaller than the predetermined brightness threshold value,the corresponding multibit word is introduced in unchanged form toconversion in activation sequences, and c) the first and second step isperformed for each sequential image point.
 2. Method according to claim1, characterized by the fact, that in the second step, when thebrightness of several sequential image points is smaller than thepredetermined brightness threshold value, a weighted mean valueformation of the brightness values of several sequential image points isperformed.
 3. Method according to claim 1 or 2, characterized by thefact, that the number of image points to be checked in the first step isat least three.
 4. Device for noise reduction in apulse-width-controlled image-display device, in which a time intervalavailable for representing an image is divided into sequential weightedpartial time intervals and the brightness signals that form a signaldata stream, that belong to the image points of the image to berepresented, and are in the form of multibit words, are produced byconversion into activation sequences assigned to the partial timeintervals, which device is composed of the following: an inputconnection (E) for receiving the signal data stream corresponding to thebrightness signals, a test unit (1) for checking if the brightness ofseveral sequential image points is smaller than the predeterminedbrightness threshold value, a switch (4) controlled by the test unit,which, on the output side, when the brightness of several sequentialimage points is not smaller than the predetermined brightness thresholdvalue, provides the time-delayed signal data stream in the unchangedform and when the brightness of several sequential image points issmaller than the predetermined brightness threshold value, provides asignal data stream filtered in the sense of a mean value formation.